Oil refineries normally include a catalytic reformer plant which aromaticizes a desulfurized naphtha cut from the crude column. Net hydrogen is generated in the process of reforming naphtha to benzene, toluene, xylene, and other aromatics. At least a portion of the net yield of hydrogen is normally used to desulfurize the naphtha and/or other streams.
The reform reactor effluent is conventionally recuperatively cooled against recycle gas and influent gas, and then cooled by ambient cooling to condense and remove liquid product. The remaining hydrogen-rich gas is compressed in a recycle compressor, and desulfurizer treat gas is withdrawn.
One problem with this conventional practice is that between 5 and 15 volume percent of the treat gas is C.sub.3 or heavier (C.sub.3 +), and would be a salable liquid were it recovered. After the desulfurization step, the C.sub.3 +is found in the desulfurizer separator off gas or the stripper off gas. Because of H.sub.2 S and H2O content, these streams are not suitable for refrigerated recovery of liquid C.sub.3 +unless preceded by H.sub.2 S and H.sub.2 O removal, and are therefore generally only suitable for refinery fuel or flaring.
There are several examples in the prior art of use of a hot hydrocarbon liquid stream withdrawn from a distillation column to power a LiBr--H.sub.2 O absorption chiller, and then use of the +5.degree. C. cooling produced by the LiBr chiller to condense the overhead vapor from the same column. See for example U.S. Pat. Nos. 2,726,519, 2,742,407, and 2,769,309. There are also prior art examples of the use of NH.sub.3 --H.sub.2 O absorption refrigeration for refrigerating hydrocarbons: both single-stage systems (U.S. Pat. No. 2,909,905) and two-stage (double-lift) systems: U.S. Pat. Nos. 2,650,480, 2,826,049, and 3,166,914. The lower refrigeration temperatures of NH.sub.3 --H.sub.2 O units compared to LiBr require that the generator temperature be correspondingly higher. Presumably that is why all the ammonia absorption refrigeration prior art disclosures for hydrocarbon processing recite steam heating or direct-fired heating, as opposed to direct heating by low temperature waste heat gas streams.
It would be desirable, and included among the objects of this invention, to recover salable liquid from desulfurizer treat gas and reformer net gas and correspondingly to increase the H.sub.2 content of the treat gas. It would be especially desirable to do so in a manner which does not require a mechanical refrigeration compressor or a turbo-expander and does not consume large quantities of electrical or shaft power. It would further be desirable to directly use low temperature waste heat to produce the refrigeration used to recover salable liquid, and to obtain the waste heat from the gas stream being treated.